Sciencemadness Discussion Board

Molecular Sieves

DraconicAcid - 15-5-2018 at 12:30

Okay- I really don't know much about molecular sieves. They're great for drying stuff, but that's about all I can tell you.

Now, I'm looking at making some esters, and I don't have a Dean-Stark trap (and they're expensive). It occurred to me that it might work to take a Vigreaux column (or similar bit of glassware), pack it with molecular sieves, and allow them to suck the water out of the system during the reflux.

Anyone who knows more about such sieves care to opine on this?

walruslover69 - 15-5-2018 at 12:58

Sieves are slightly basic and will be attacked by acid so it probably isn't compatible for an acid catalyzed esterification.

DraconicAcid - 15-5-2018 at 13:01

Yes, but they'll be in the column, where they'll be in contact with the alcohol and water vapours, not the non-volatile catalyst acid.

zed - 15-5-2018 at 13:54

The Dean-Starks I have seen, seem fragile and expensive. Also, they depend on using a solvent immiscible with the water being removed. The Soxhlet can generally do the same thing, but it has further applications.

https://www.youtube.com/watch?v=Ah5ds_3s5BI

500 ml, is actually a pretty big unit.

https://www.ebay.com/itm/500ml-24-40-Glass-Soxhlet-Extractor...

Might be too big for the scale of experiment you are planning.

Smaller units are less expensive.

[Edited on 15-5-2018 by zed]

Tsjerk - 16-5-2018 at 01:11

This has been done before... If you would use sulfuric acid or so as catalyst it should work fine.

https://www.google.nl/search?rlz=1C1EJFA_enNL769NL769&q=...

walruslover69 - 16-5-2018 at 14:18

It would probably work the way you described but isn't optimal.

1) drying by Just packing a reflux column with sieves is going to be very slow. In a soxhlet you are cooling and condensing the vapor. Sieves work slower at higher temperatures so they will absorb water slower in a reflux column. In a soxhlet the sieves are allowed to sit in the condensed liquid vs having the condensation constantly dripping down in your reflux column, slowing adsorption even further.

2) You still might have problems with your carboxylic acid dissolving the sieves. Carboxylic acids smaller than pentanoic acid have a azeotrope with water with a boiling point of around 100C, this is higher than the water/alcohol azeotrope but only by 10-20 degress so without a fractional coloum the vapor and condensate you are passing over your sieves contains acid. In a soxhlet the condensate is cooled to room temperature acid reacts very slowly, but with a reflux column your sieves will be in constant contact with a dilute solution of boiling acid. I doubt it would harm your reaction but its a waste of good sieves.

3) would it not just be more efficient to just ad an excess of cheap sulfuric acid? anhydrous copper sulfate would also make a faster and superior drying agents to pack a reflux column with.

DraconicAcid - 16-5-2018 at 14:27

The trouble with using other drying agents such as copper(II) sulphate or calcium chloride is that the column wouldn't stay packed with it- they'd dissolve at least somewhat in the partially aqueous mix and fall into the reaction mixture.

walruslover69 - 16-5-2018 at 14:31

That hadn't occurred to me. Would that really be a problem though? I imagine you are doing to distill your product anyways. Thinking about it now, I don't see a problem with just adding your drying agent directly. The only reason sieves aren't added directly is because of the acid.

What kind of esters are you looking into making? just out of curiosity.

Waffles SS - 16-5-2018 at 18:50

I usually use this system for making ethyl ester of different carboxylic acids.Also this is possible to use it for another alcohol if you can make azeotrope for bring out of formed water.



[Edited on 17-5-2018 by Waffles SS]

[Edited on 17-5-2018 by Waffles SS]

2.jpg - 61kB

DraconicAcid - 16-5-2018 at 20:15

Yeah, I'd go with the addition funnel if I had one, but right now I'm trying to make do. And addign the drying agent right to the reaction mixture won't work as well, because it will be at a temperature where it's not absorbing water very well. I'm mostly looking at benzoates and salicylates, which have too high bp to effectively distill.

Waffles SS - 18-5-2018 at 09:12

What ester do you want to make?

Maybe there is better and cheaper method for synthesis it

DraconicAcid - 18-5-2018 at 09:15

MAKE ALL THE ESTERS!!!

That basically sums it up......

Waffles SS - 18-5-2018 at 09:31

Usually adding excess sulfuric acid as Esterification catalyst and water absorber work fine for synthesis most of esters. I made many esters by this mothod.
Below link is synthesis of Dimethyl oxalate by this method.

http://www.orgsyn.org/demo.aspx?prep=CV2P0414

But if you are still interest to use molecular sieve then i think there is certain type of it that is resistance to acids.just search it




[Edited on 18-5-2018 by Waffles SS]

DraconicAcid - 18-5-2018 at 10:17

Yeah, but the more esters I try, the more I find that things that should work fine don't really seem to work for me.....

zed - 18-5-2018 at 14:10

I have thoughtfully designed and executed a lot of clever experiments, in numerous areas.

At least, I thought they were clever, at the time.

I almost always failed.



DavidJR - 18-5-2018 at 16:50

You don't need to buy an actual dean stark trap. I use a few adapters that I already had to achieve the same goal (and more flexibly than if i'd bought a real dean-stark trap). Basically, a bent adapter from the boiling flask, which goes to the side arm of a still head/3-way adapter. Reflux condenser connected to the top of the 3-way adapter and a small flask to the bottom, where the boiling flask would normally go. You can also hook up a small sep funnel, a hose connection w/ stopcock, etc.

aga - 19-5-2018 at 01:17

Quote: Originally posted by DraconicAcid  
Yeah, but the more esters I try, the more I find that things that should work fine don't really seem to work for me.....

If you pick something simple, like wintergreen, i'll have a go at replicating your setup and see what happens.

Hardly any -ols here tho.

SWIM - 19-5-2018 at 09:13

Quote: Originally posted by DavidJR  
You don't need to buy an actual dean stark trap. I use a few adapters that I already had to achieve the same goal (and more flexibly than if i'd bought a real dean-stark trap). Basically, a bent adapter from the boiling flask, which goes to the side arm of a still head/3-way adapter. Reflux condenser connected to the top of the 3-way adapter and a small flask to the bottom, where the boiling flask would normally go. You can also hook up a small sep funnel, a hose connection w/ stopcock, etc.


I have done similar by just using the distillation head on the neck of the flask, with a condenser where the thermometer normally goes, and a flask where the condenser would normally go.

The flask has to be at an angle so the collection flask is down and the condenser up, but it does work. The distallate runs down the angled condenser into the collection flask and overflows back to the reaction pot.